He was raised in an Orthodox Jewish family and his father was Rabbi Solomon Greenstein, who opposed Oswald Mosley's British Union of Fascists at the Battle of Cable Street in 1936.
In 1974, he became involved in housing activism alongside Steve Bassam and squatted in derelict hotels before negotiating a licence to live at Lansdowne Place.
In more recent years he has been involved in campaigning against far-right groups such as the British National Party, English Defence League and March for England (MfE) in the city.
[15] In December 2021 Greenstein accepted a two-year restraining order in return for the Crown Prosecution Service dropping two charges of harassment against the Labour party's disputes team.
[16][17] In September 2023 Greenstein was given a 9 month prison sentence, suspended for two years, for his part in a Palestine Action attack on the factory of Elbit Systems, an Israeli arms company, at Shenstone near Walsall.